Space discharge device



SPACE DISCHARGE DEVICE Filed Aug. 245, 1942 ATTORNEY Patented Feb. 22, 1949 UNITED STATES PATENT orifice SPACE DISCHARGE DEVICE Leland A. Wooten, Maplewood, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York l Application August 25, 1942, Serial No. 456,004

(Cl. Z50-165) 7 claims.

This invention relates to space discharge de- `vices and more particularly to photoelectric tubes.

a photoelectric tube comprises an evacuable glass container, a plurality of electrodes and a caesium producing pellet containing an intimate mixture .of nely divided caesium chromate, chromic oxide, aluminum and cuprous oxide. In the process of sensitizing one of the electrodes to render it photoelectrically emissive, the container is evacuated, the electrode to be sensitized is prepared for treatment with caesium and the caesium producing pellet is heated to initiate an exothermic reaction which is carried to completion by the heat produced by the reaction and all of the caesium is released in the form of vapor. The uniformity of this reaction is greatly improved by the presence of the cuprous oxide. It is believed that the cuprous oxide enters into the reaction to produce the improved results in two ways. The first elect is to reduce the amount of initial heating required to start the `reaction over that which would be required if the cuprous oxide were not mixed with the other ingredients because cuprous oxide reacts with nelyA divided aluminum at a lower temperature than does chromic oxide. The reaction of cuprous oxide with the aluminum is an exothermic reaction which produces additional heat to facilitate further the exothermic reaction of the chromic oxide with the aluminum. The second effect, probably the more important of the two,

is to keep the surfaces of the aluminum particles clean throughout the reaction by the action of the free copper produced by the reduction of the cuprous oxide. The copper may prevent the aluminum particles from becoming coated with a protective film of aluminum oxide possibly by formation of a copper-aluminum alloy, or the copper may serve as a catalyst for the reaction. Whatever may be the' true explanation the presence of the cuprous oxide in the reactive mixture improves the reaction so that all of the caesium originally in the form of caesium chromate is uniformly released in the form of caesium vapor.

'I IC'his makes possible the manufacture of photo- "4 electric tubes with a small percentage of defective tubes with resulting improvement in economy of production.

Other metallic compounds of metals more positive than aluminum in the electromotive force series may be used in place of caesium and other compounds which enter into an exothermic reaction with aluminum may be used in place of the chromates of such metals and ychromic oxide for certain purposes. 'Ihe other alkali metals, the alkaline earth metals and magnesium may be specifically mentioned.

This invention will now be described in rnore detail having reference to the accompanying drawing. Fig. 1 is a longitudinal section of an embodiment of the invention in a structure suitable for conversion into a photoelectric tube.

Fig. 2 shows a mounted caesium producing pellet according to this invention.

Like reference characters are used to refer to identical elements in the gures of the drawing. The embodiment of the invention illustrated in Fig. 1 is a so-called seal-in for a photoelectric tube. The seal-in, as the term is used in this specification, is a fabricated tube structure ready to be put through a sensitization process which converts the seal-in from a nonphotoelectric device to a photoelectric tube. The seal-in of Fig. 1 comprises a glass container 3 consisting of a generally cylindrical portion 8 and a molded dished stem `li joined by a ring seal 1, anelectrode structure fl supported from the molded dished stem 6 and a caesium producing pellet 5 suitably mounted in proximity to the electrode structure. Pellet 5 is composed of an improved mixture of chemical ingredients which react with an exothermic reaction to produce a uniform amount of caesium vapor within the evacuated container 3. One such improved mixture comprises caesium chromate (Cs2CrO4),

, chromic oxide (Crz03) and an active 'cuprous oxide-aluminum mixture which latter mixture consists of commercial powdered aluminum (Al) thoroughly mixed with a small per cent of cuprous oxide (CuzO) by rolling in a ball mill using flint pebbles. The amount of ycuprous oxide may be varied within rather wide limits extending from 0.10 per cent 'to one per cent of the amount of aluminum powder by weight. Very good results are obtained by mixing with the aluminum powder an amount of cuprous oxide equal to 0.5 per cent of the weight of the aluminum powder. A suitable mixture for making `caesium. producing. pellets comprises five grams f shielding rim II of Cs2CrO4, 31.8 grams of CrzOa and 14.212 grams of an active cuprous oxide-aluminum mixture. The amount of active cuprous oxide-aluminum mixture used may be varied considerably provided that there is suiiicient aluminum present to completely reduce the caesium chromate and chromic oxide. A suitable mixture expressed in per cent of the ingredients by weightf'c'omprises 9.8 per cent of caesium chromate, 62.4 per cent of chromic oxide, 27.62 per cent of powdered aluminum and 0.138 per cent of cuprous oxide, al1 of said ingredients being nely divided and thoroughly mixed.

The caesium producing mixture is formed into pellets by pressure in a mold. These pellets are then mounted to form part of the 'seal-in illustrated in Fig. 1. The electrode structure 4 comprises a cathode I which is formed advantageously from substantially pure silver Ain the shape of a frustrum of a cone, the large end of the cone being spun over'to form a focussing and The convex conical surface of the cathode I0 in the completed photoelectric tube is treated to make it'photoelectrically sensitive. The small Vend of the 'cathode I0 is turned .iniaga-inst one surface of a mica washer I2, the

other surface of which nts against `an annular metallic `shield I3. The cathode I0, washer I2 and shield I3 are held together by a long shank rivet III, nickel washer I5 and nickel clamping washer I6. 'I'he washer I6 is forced over a beveled section of the shank of rivet I 4 thereby lproviding a tight t'and a portion of the beveled section is sheared down and forced over the 'washer I6 by the riveting tool. A mica disc I7 provided vwith a center eyelet I8 Vrests on the spunover end of cathode I0, the'shank of rivet I4 passing through kthe'eyelet I vBand being welded thereto. Shield I3 is supported on four lead-in conductors which are sealed through the dished stem `6 at positions 'equally spaced onfa circle centered at the center of the stem. Conductors 20 and 22 only are shown inthe drawing. The anode 28 is a `nickel ring 'of circular cross section supported von :four lead-in conductors also sealed through the dish lstem 6 at :positions on `the same circle as the conductors supporting the shield I3 but midway between those conductors respectively. Three of y'these lead-in conductors 2`I, 23 and 25 .areshown in the drawing. The anode 28 is so .positioned with respect to the cathode I0, .the shielding rim 'II and the shield I3, that the electrons emitted from the surface ofthe cathode I 0 are focussed on the anode 28 and prevented from accumulating onthe glass walls of the container 8 and otherinsula'ting surfaces to cause noise during the operation of the tube. The

shape of the cathode I0 and shield I3 is such'as to facilitate focussing.

kThe caesium producing 'pellet 5 .is :mounted 'in a holder -30 shown '1in perspective in Fig. 2 comprising a vnickel `disc 3l and nickel-gauze 'disc '32 welded together ia't four equally spaced positions 'around the lperiphery `of the discs, two 'of these welds lat diametrically opposite Ypositions includ- 'ing support wires 33. The v'gauze disc 32 is shown partially cut away -in Fig. 2 to expose the vpellet 5. The gauze disc 32 is resilient and when welded to the disc 3| holds the pellet 5'f1rmly against the -disc 3 I. The support wires 33 are welded respectively tothe anode support wires 2'I and 25 transversely to the 'axis of the seal-in fand a'small distance from the dished s'tem yIi. In order that the .length of the `container 3 may be made as small as possible, pellet holder v3l! is located very Vclose 4 to the dished stem 6. In fact, it is placed so close that the stem would be injured if means were not provided to prevent such injury during the chemical reaction of the caesium pellet 5. To prevent injury another mica disc 35 is supported from the anode support wires through the intermediary of eyelets 36 through which the support wires pass and yto which they are Welded. The cathodesupport wires pass through l`notches in the edge of disc 35 and do not touch the disc.

The seal-in is completed by the glass portion `8 -which covers the electrode structure and is joined to the glass stem 6 at the ring seal I. The portion v8 is pressed down so that the edges of mica disc I I contact the inner surface of the portion 8 and are slightly depressed thereby. Exhaust tubulation 9 is adapted to be connected to a suitable pump station of well-known type during the 'sensitizing process when the seal-in is converted into a photoelectric tube.

Cathode I0 vmay Abe sensitized in any well- .known manner after the seal-in has been fabricated. Brieiiy described an advantageous method of sensitizing the cathode is as follows:

The seal-in is baked in an oven at about 400 C. to remove occluded gas from the bulb but this heating does not cause any chemical reaction in the caesium pellet 5. Oxygen is admitted into the container and the cathode is subjected Ato ionic bombardment in such a way as to produce a'heavy layer of silver oxide lon the convex surface of the conical cathode I0. 'Ihe residual oxygen is then pumped out and the cathode inductively heated by means of a high frequency coil surrounding the container `to reduce the heavy layer of silver oxide. This cycle of oxidation and reduction is repeated. VThis treatvment leaves the convex surface -of the cathode I0 clean and slightly rough so that it has a uniform matte. surface. The surface of the nickel shield I3 'remains unchanged. A fresh charge of oxygen is admitted and by a succession of discharges of fixed amounts of electricity from condensers the convex surface of the cathode is oxidized to an amount depending upon the amount of caesium to be introduced. The mica disc I1 functions to prevent discharges to the inner surface of the conical cathode VI 0 andA thus facilitates the oxidation of the convex surface thereof. The caesium pellet 5 is then inductively heatedto effect a chemical reaction which produces caesium vapor. This is commonly called dashing the caesium. The Ahigh frequency coil used for linductively heating the caesium pellet holder 30 -is positioned around the vstem 6 with the upper vend of the coil at approximately the level of Vthe holder 30. In order to protect the cathode I0 from induced currents a short-circuiting turn in the form of a short length of copper tubinggis placed `around the cylindrical rportion '8 of the 'container with the `lower end at the level Vof the shield I3. A certain amount of heating Aof the shield `I`3 takes place nevertheless but such-heating'is prevented from injuring the oxidized cathode II) by the mica disc I2 which greatly reduces the conduction of heat from the shield -I3 to the cathode I0. Caesium vapor produced by the chemical reaction condenses primarily on the innerwall of the glass container and is prevented from directly striking vthe oxidized surface' of the 'silver cathode I0 by the shield I3. The container is then heated in a stream of hot air to a temperature of Vabout '225 C. until the ycathode vhas :reached the desired sensitivity. Argon or 'other suitable gases lmay `be admitted at low pressure to obtain the benefits of gas amplification. A modied method of sensitization may be used in which argon or other suitable gas is admitted immediately after hashing the caesium and the tube sealed off. A subsequent heat treatment, dissociated from the pump station completes the sensitization.

The amount of caesium producing mixture which is used in the seal-in depends upon the amount of caesium required to produce the desired sensitivity. The cuprous oxide may be mixed with the other ingredients caesium chromate, chromic oxide and powdered aluminum simply as an added ingredient but best results have been obtained by mixing in the manner hereinbefore described namely by mixing the cuprous oxide with the aluminum powder iirst to produce an active cuprous oxide-aluminum mixture and then mixing the active cuprous oxide-aluminum mixture with the other ingredients, caesium chromate and chromic oxide. The metal producing mixture may be mounted in a side container connected to the seal-in in such a manner that both the seal-in and the side container may be evacuated through a single pump connection.

The outstanding advantage of this invention is the uniformity of the reaction of the caesium pellets, which uniformity it is believed is made possible by the action oi the free copper in keeping the surfaces of the aluminum particles in condition for reaction with the chromic oxide and caesium chromate until the reaction is complete. All of the caesium in the caesium chromate is thus released when the pellet is flashed and since the amount of caesium chromate can be closely controlled the amount of caesium produced is also closely controlled. This close control of the amount of caesium produced makes for uniformity in the sensitivity of photoelectric tubes produced by the use of this invention.

The use of an active cuprous oxide-aluminum mixture is of general application in the production of metallic vapors by chemical reactions in which aluminum powder is used as the reducing agent.

What is claimed is:

1. A caesium producing mixture comprising a caesium compound which reacts with powdered aluminum when heated, powdered aluminum and cuprous oxide.

2. A caesium producing mixture comprising iinely divided and intimately mixed caesium chromate, chromic oxide, cuprous oxide and aluminum.

3. A caesium producing mixture comprising a compressed pellet of finely divided and intimately mixed caesium chromate, chromic oxide, cuprous oxide and aluminum.

4. A caesium producing mixture comprising nely divided and intimately mixed caesium chromate, chromic oxide, cuprous -oxide and aluminum wherein the amount of cuprous oxide is within the limits of 0.10 per cent and one per cent of the amount of aluminum by weight.

5. A caesium producing mixture comprising by weight 9.8 per centof caesium chromate, 62.4 per centof chromic oxide, 27.62 per cent of powdered aluminum and 0.138 per cent of cuprous oxide all of said ingredients being iinely divided and thoroughly mixed.

6. A method of sensitizng an electrode within an evacuated container which comprises providing within said container an electrode to be sensitized to emit electrons when externally excited and a.mixture of caesium chromate, chromic oxide, cuprous oxide and aluminum in proportions to produce an exothermic reaction, and heating the mixture to the reaction temperature of said mixture producing caesium to sensitize said electrode.

7. A method of producing a caesium producing mixture which comprises thoroughly mixing finely divided aluminum and less than one per cent and morethan one-tenth per cent by weight of cuprous oxide in a ball mill using quartz pebbles to produce an active cuprous oxide-aluminum mixture, and subsequently mixing a nely divided caesium containing compound with said active cuprous oxide-aluminum mixture.

LELAND A. WOOTEN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,707,637 Miesse Apr. 2, .1929 1,733,809 Lederer Oct. 29, 1929 1,797,131 De Boer Mar. 17, 1931 1,835,118 Marden et a1. Dec. 8, 1931 1,837,746 Zworykin Dec. 22, 1931 '1,860,187 Koller May 24, 1932 1,861,637 Marden June 7, 1932 1,991,774 Spencer Feb. 19, 1935 2,003,344 De Boer et al. June 4, 1935 2,236,647 McIlvaine Apr. 1, 1941 

